It's quite simple to sex them, I assure you.
Look at the large belly area. Just behind it, on the bottom of the fish, there is a small fin (the anal fin, for future reference). In females, it is fan-shaped. In males, it is long, slender, and elongated, and serves much the same function as a penis in mammals.
If they are a female, and they have ever been in the presence of a health, mature male Molly...they're pregnant. It's pretty much that easy. You can tell how far along they are by how big their belly gets. When they get close to birthing, they will appear in a front view to be rather squarish, and will become shy and reclusive. They will also have a tendency to hide, and chase off all other fish from their hiding place.